A common method of maximizing the thermal insulation ability of panels and windows made of glass, glass laminates or plastics is to provide a "dead-air" space between individual panes. Multiple pane glass windows increase the insulating ability of typical glass windows. The thermal resistance, or "R-value" measured in .degree.F.-hr-ft.sup.2 /Btu, is the mean temperature difference at equilibrium between two defined surfaces of material that induces a unit heat flow rate through a unit area. Heat flow (Q) as measured in Btu/ft.sup.2 -hr is related to the difference in temperature (.DELTA.T) between the interior and exterior of a window or similar structure roughly in accordance with the following general insulation equation: EQU Q.sub.heat flow =.DELTA.T/R
Thermal resistance, or R-value, exhibits an inverse relationship to heat flow. Therefore higher R-values, exhibit less heat flow and represent better insulation properties.
For example a single-pane window with an approximate thickness of about 1/4 inch has an R-value of about 0.96.degree. F.-hr-ft.sup.2 /Btu in relation to summer heat gain and an R-value of about 0.88.degree. F.-hr-ft.sup.2 /Btu in relation to winter heat loss. By providing two panes of glass with an intervening dead-air space of about 1/2 inch, the R-value is raised to about 1.79 in the summer and about 2.04 in the winter. However, multiple layers of glass significantly add to the weight and cost of a window. In addition, glass typically allows direct, intense light as opposed to a softer diffused light to pass through.
To decrease cost and weight, but to maintain or improve thermal resistance, polymeric/glass laminates and polymeric panels were developed. These laminates and panels typically utilize polymeric structures which have a single layer of void spaces between two plates. Polycarbonate panels, for example LEXAN.TM. manufactured by General Electric Plastics, at 1/4 inch thickness are approximately 90% lighter than glass and have R-values in the range of 1.56 in relation to summer heat gain and 1.54 in relation to winter heat loss. R-values and weight are particularly important in the manufacture of panelized skylights, greenhouses or similar rooms or other window wall structures. In addition, polycarbonate-based panels have the added advantage of an extremely high impact resistance. LEXAN.TM. polycarbonate sheet has a Notched Izod value at 1/8 inch of from about 12 to 16 ft-lbs/in.sup.2.
To further improve the insulation properties of polymeric panels, there have been unsuccessful attempts to make a panel having more than three plates such that more than two layers of void spaces may be provided between the plates. These panels have only two layers of individual voids and little structural integrity due to the large volume of each of the individual void spaces and problems which were encountered in successfully extruding such a panel.
Typically such panels are interconnected by an aluminum framing system. To allow for thermal expansion, space is left within the aluminum frame and the frame is sealed. The panels within the structure are typically manufactured in lengths of up to 12 feet. Aluminum framing is required around each panel and around the periphery of the structure. This represents a significant amount of aluminum framing both for structural support and to compensate for thermal expansion. Such framing has attendant fabrication and raw material costs which are incorporated into the overall cost of a typical synthetic resin panel system. In addition, large amounts of aluminum framing significantly increase the overall weight of the structure which must be supported. These panels also require exterior capping which often leaks.
Therefore a need in the art exists for a synthetic resin panel having improved insulation capability which does not appreciably increase the weight of an individual panel, and the attendant costs of manufacturing. In addition, there is a need for an improved multiple synthetic resin panel system which has sufficient internal structural support and room for thermal expansion without incurring the high fabrication and raw material costs associated with typical aluminum framing systems and which does not require horizontal capping, controls the flow of water and virtually eliminates the potential for water infiltration.